1. Field of the Invention
The present invention relates to a measuring apparatus and, more particularly, to a measuring apparatus for radiating light in blood to measure e.g., the oxygen saturation of the blood in accordance with a reflected light intensity.
2. Description of the Related Art
In recent years, a system or apparatus has been developed in which light is radiated in blood through an optical fiber incorporated in a cardiac catheter, and the intensity of a light component of all the radiated light component, which is reflected by the blood, is measured to monitor the oxygen saturation (SVO.sub.2) of a mixed venous blood in accordance with the light-absorbing characteristics of hemoglobin. This oxygen saturation monitor for the mixed venous blood plays an important role in detecting and alarming an abrupt change in operating states of circulatory organs in patient's management upon a direct vision intracardiac operation. In addition, this oxygen saturation monitor is useful in evaluating a certain load on a living body, such as drug administration.
When the oxygen saturation is to be measured using such a system or apparatus, various factors except for the oxygen saturation in blood are considered as factors for varying the intensity of light reflected by blood. It is important to suppress the influences of these factors. Some factors may be based on a measuring system, but variation factors derived from a living system are more important. It is, therefore, essential to correct measurement data varied by these variation factors in development of a monitor system.
A system for measuring and monitoring the oxygen saturation will be exemplified. To continuously monitor the oxygen saturation in human blood, light components having two specific wavelengths are radiated in blood using a catheter or the like, and back-scattered light components (reflected light components) from the blood are detected. The specific wavelengths here are defined as a wavelength (isosbestic wavelength: 805 nm) at which the absorbency of oxyhemoglobin is equal to that of reduced hemoglobin and a wavelength (660 nm) at which a difference between the absorbencies thereof becomes large. An intensity signal of reflected light having a wavelength of 805 nm as the isosbestic wavelength rarely depends on the oxygen saturation. However, an intensity signal of reflected light having the wavelength of 600 nm greatly changes depending on the oxygen saturation. These two signals are compared to obtain the oxygen saturation of blood.
An equation for calculating the oxygen saturation during its continuous monitoring was reported by Polanyi et al. in 1960 as follows: EQU Oxygen Saturation=A+B.times.(Reflected Light Intensity of Near-Infrared Ray)/(Reflected Light Intensity of Red Light)
where A and B are the constants.
In this equation, a ratio of the reflected light intensity at one wavelength to that at the other wavelength is calculated. For this reason, influences such as a change in blood velocity and sizes of blood cells can be canceled to a considerable degree. However, influences on these reflected light intensities by the hematocrit value are different from each other. For this reason, the influences by the hematocrit value cannot be canceled each other even if their ratio is obtained. Therefore, the above equation cannot be expected to obtain a sufficiently high precision.
The following equation is used for hematocrit correction:
______________________________________ Oxygen Saturation = A' + B' .times. (Reflected Light Intensity of Near-Infrared Ray + Correction Term 1)/Reflected Light Intensity of Red Light + Correction Term 2) ______________________________________
where terms 1 and 2 are values experimentally determined to minimize the influences of the hematocrit value, and A' and B' are constants.
This calculation method is effective in monitoring using a sensor probe fixed to a blood circuit such as extracorporeal circulation. However, an SVO.sub.2 monitor probe is generally incorporated in the form which adds a function to a catheter for measuring a cardiac minute volume, like a thermal dilution method (so-called Swan-Ganz method). Therefore, the position of a probe sensor is not fixed in a pulmonary artery. The measured intensity of light reflected by blood may include light reflected by the wall of a blood vessel and a valve in an unknown proportion.
This possibility will be considered in detail. A radiating portion for radiating light for measuring the oxygen saturation in blood, and a portion for receiving light reflected by the blood are generally constituted by optical fibers. A fiber opening for radiating light and a fiber opening for receiving reflected light are formed at the distal end of a probe. When the distal end portion of the probe comes close to the wall of the blood vessel at an angle of almost 90.degree. with respect to the longitudinal direction of the catheter, the opening for receiving reflected light receives light reflected by the blood and light reflected by the inner wall of the blood vessel, thereby increasing the intensity of the reflected light. In many cases, the detected intensity of the reflected light may greatly vary in synchronism with a blood flow and respiration. In addition, when the probe comes close to or is brought into contact with the wall of the blood vessel in a state wherein the distal end portion of the probe is held at an angle of almost 180.degree. with respect to the longitudinal direction of the catheter, most of the light reflected by the wall of the blood vessel is reflected forward. For this reason, the intensity of light reflected by the wall of the blood vessel and incident on the fiber opening for receiving the reflected light is very low. The variations in the reflected light intensity synchronized with the blood flow and respiration may be extremely attenuated.